1. Field of the Invention
The present invention relates to balanced differential amplifiers, that is, to amplifiers having two differential outputs, a first signal and a second signal, the two signals being in phase opposition.
The invention more particularly relates to a device for regulating the common mode voltage at the output of such an amplifier.
2. Discussion of the Related Art
Balanced amplifiers are used when a high variation of output voltage (output dynamic) is needed, but when only a low supply voltage is available.
FIG. 1 schematically represents a balanced amplifier 10. Amplifier 10 includes two differential amplifiers 12 and 13, each powered by a high voltage Vcc and a low voltage GND. The outputs of amplifiers 12 and 13 form the differential outputs of the balanced amplifier 10. The inverting inputs of the differential amplifiers 12 and 13 connect to differential input terminals, E+ and E-, respectively.
The common mode voltage VCM at the output of amplifier 10, that is, the average of the output voltages of amplifiers 12 and 13, may vary. A first cause of variation is a change in the common mode voltage at the input, which occurs, for example, if the voltages at terminals E+ and E- are not symmetric with respect to a fixed common mode reference voltage. A second, systematic, cause of variation is a change in the characteristics of the N-type and P-type transistors that form amplifier 10.
A high variation of voltage VCM with respect to a reference value, (Vcc+GND)/2, for example, can cause clipping of the output signals of amplifier 10, particularly if the output signal amplitude is close to its maximum value. For this reason, it is desirable to regulate the output common mode voltage VCM.
FIG. 1 shows a conventional device for regulating the common mode output voltage of a balanced differential amplifier. This device includes a corrector differential amplifier 15, whose output connects to the non-inverting inputs of amplifiers 12 and 13. The inverting input of corrector amplifier 15 receives VCM, the mean value of the output voltages of amplifiers 12 and 13, through a pair of resistors 17 and 18 having the same resistance value. The non-inverting input of corrector amplifier 15 receives a reference voltage VCM.sub.0, the value with respect to which the corrector amplifier 15 regulates the common mode voltage VCM.
With this configuration, any resulting common mode errors, that is, differences between voltage VCM and voltage VCM.sub.0, are inversely proportional to the gain of the corrector amplifier 15. Thus, errors are minimized if the gain is very high.
Conventionally, since regulation must be stable and fast, amplifier 15 comprises a single differential stage.
In order to limit the power consumption of this stage, its polarization current is chosen to be low. However, low polarization current causes the stage to saturate for high values of common mode error. When saturation occurs, the gain of the differential stage significantly drops, thus impairing the stages ability to correct high common mode errors. Since the polarization current of the stage is chosen to be low, the stage rapidly saturates. Thus, there is a trade-off in the prior art between low power consumption in corrector amplifier 15 and high common mode voltage correction range.